1. Field of the Invention
This invention relates to a method for recovering flux fines used in a metallurgical process, and to equipment used in such a process.
2. Description of the Prior Art
It is not uncommon to operate an oxygen-steelmaking vessel without any attempt to recover and use the fines contained in the limestone or other flux fed to the process. In some cases, the fines amount to less than 10 weight percent of the as-received material, the material being relatively dense and non-dusty. Such material, while desirable from the standpoint of avoiding problems of air pollution and loss of flux values, has a drawback, in that its use is associated with a need for longer blowing times per heat and a lowering of the throughput of the process. It is more usual for the as-received material to be somewhat softer and to contain a substantial proportion of fines, such as 20 to 60 weight percent.
In oxygen steelmaking, those skilled in the art know that fine flux particles do not contribute to the flux available to the bath of molten iron in the vessel. The molten iron is at a temperature such as 2900.degree. F., and it radiates heat to the air in the vicinity of its surface at a rate such that a strong updraft is created, and only flux particles of some substantial size can pass through the updraft. Fine particles in the as-received limestone, if not incidentally lost elsewhere in the process of receiving the flux and conveying it to the vessel, are rejected into the ambient air when the fines-containing flux is fed to the vessel, for reasons indicated above.
Flux fines rejected into the air via updraft (or otherwise lost in transport) are not at all commonly recovered in any useful form. By the time that they settle, they are admixed with various other airborne impurities. Prior to the present invention, such fines have constituted a waste material, one entailing expense for clean-up of the vicinity of the process vessel and further expense for ultimate disposal (hauling and dumping). That is not all: there is further expense in connection with providing the necessary personnel-protection equipment to deal with a dust-laden working environment. Moreover, the fines sometimes work their way into, for example, the bearings of the conveyor equipment and cause shutdown because conveyor rolls seize.
Those skilled in the art have naturally considered the approach of providing the conveyors with hoods and using blowers to force the airborne fines to enter collecting equipment. This approach is not feasible because of the great expense of erecting operating and maintaining such equipment and because of the failure of such an approach to deal with the problems posed by the particles of flux which do not become airborne before they encounter the updraft created by the air in the process vessel.
Of course, the approach whereby the process vessel itself is hooded and its entire gaseous output is collected and, after processing, stored in a large, expandable vessel, is also known (see U.S. Pat. Nos. 2,829,960 and 2,855,292), but to the applicant's knowledge such a system has never reached the state of commercial feasibility.
The idea of recovering airborne fines with the use of blower means plus baghouse means is old in the art. It has not, however, been obvious to those skilled in the art of operating metallurgical-process vessels in general, and oxygen steelmaking vessels in particular, that there would be an economic benefit from the use of such means that would justify the cost of employing them. The use of such means in a flux-recovering zone to recover airborne flux particles in that zone, apart from a process which makes the recovery and use of the flux fines practical, is not an economically attractive proposition, because it solves only a part of the problem--there are other fines which are produced in the process of conveying the flux to the vessel, and there are fines which are not airborne in the receiving zone but become airborne when an attempt is made to feed them to the molten-iron-containing vessel. Attempts to remove the fines which are likely to become airborne at that time, as by screening, also are not economically attractive, because screening adds to costs and it further degrades the size of the flux particles and creates more fines and more dust.
It may be taken as known that flux particles, such as finely divided limestone, can be agglomerated by briquetting. The idea of using briquetting as a way of turning limestone fines into lumps of flux material that are useful in an oxygen steelmaking vessel or similar metallurgical process is an old one, but the practical results with many of such processes have been disappointing for various reasons. It has been found that some briquetting processes, ones which do not employ suitable conditions and procedures for the briquetting process, yield briquettes which cannot be handled or stored without undergoing degradation to an intolerable degree. There has been some question whether, considering the pressures required in order to obtain briquettes of satisfactory handling and storage characteristics, the costs of briquetting can be justified, considering the cost of the wear-resistant material needed to make the briquetting rolls and the service life that can be expected from such rolls under the operating conditions. It can, however, be taken as known, as of the time of making of the present invention, that it is physically possible to convert limestone fines in a binderless process to briquettes having tolerable storage and handling characteristics, especially if the "flashing" problem could be overcome or ignored.
Those skilled in the art of agglomeration know that every briquetting process involves to some extent a problem in that the briquette produced has, at a location corresponding to the place at which the briquetting rolls meet in the formation of the briquette, a ridge of "flashing." The flashing is relatively weak and constitutes a protuberance extending outward from the general contours of the briquette; as such, it is subject to becoming removed, for example, in handling, by contact with conveying means or other briquettes. It is desirable to have a process in which problems caused by flashing are dealt with.
Another point to be considered is the fact that briquettes made of limestone fines often do not exhibit good storage characteristics. Although such briquettes, when freshly made, can be handled with no particular difficulty, this does not continue. The briquettes tend to absorb moisture from the air and to become swollen and softened, whereupon they cannot be handled further without degrading and producing fines. This is a further reason for those skilled in the art to look away from briquetting and towards some alternative means of agglomeration.
The other equipment (dense-phase pneumatic conveyor, screen, weighing means) used with the present invention has been known per se. What has not been known or appreciated is that such equipment may usefully be combined with the other equipment discussed above to yield a working ensemble which uses briquetting and deals adequately with the flashing problem, so that relatively freshly formed flux briquettes are fed to a metallurgical process, and at a considerable savings in cost. This means that the investment in the equipment required is rapidly recovered, and the problems associated with use of a material such as soft limestone containing 20 to 60% of fines in an oxygen steelmaking furnace are overcome.